1. Field of the Invention
The present invention relates to a method of forming an overcoat member of a polymer insulator having a core member and the overcoat member made of a sheath portion arranged around the core member and a plurality of sheds, in which the overcoat member of the polymer insulator is formed by using a metal mold.
2. Description of Related Art
FIG. 7 is a partial cross sectional view showing one embodiment of a known polymer insulator which is also a target of the present invention. In the embodiment shown in FIG. 7, a polymer insulator 1 comprises a solid FRP core 2 as a core member, an overcoat member 3 made of rubber such as silicon rubber arranged around the FRP core 2, and metal fittings 4 fixed to both end portions of the FRP core 2. The overcoat member 3 has a sheath portion 5 and a plurality of sheds 6.
The overcoat member 3 of the polymer insulator 1 is generally formed by a metal mold consisting of an upper mold and a lower mold. When the overcoat member 3 is formed by using the metal mold, a thickness of the sheath portion 5 is not maintained constantly even, if the FRP core 2 is eccentric. If such a polymer insulator 1 is used, the sheath portion 5 is sometimes penetrated, when a flashover due to for example an abnormal voltage of power transmission line is applied thereto. In forming operations using the metal mold such as compression forming, transfer forming, injection molding and so on, such an eccentric FRP core 2 likely occurs, and thus countermeasures are required.
As the countermeasure in the transfer forming or the injection forming process, there is known a method, as shown in FIGS. 8a and 8b with a plan view and a front view, comprising the steps of: arranging support rod 3 for supporting the FRP rod 2 with respect to a rubber flow supplied through a runner 51 and nozzle 52; filling rubber materials in a cavity 55 of a metal mold 54 through the runner 51 and the nozzle 52 under such a condition; and removing the support rod 53 from the cavity 53 by moving the support rod 53 backward. Moreover, in order to prevent a flaw generation on the FRP core 2 by the support rod 53 when using the above method, there is known an additional method, as shown in FIGS. 9a and 9b with a plan view and a front view, comprising the steps of: winding a ring 56 around the FRP rod 2, which is made of the same rubber as that of the FRP core 2; and arranging the support rod 53 via the ring 56.
The known countermeasure mentioned above is slightly effective for preventing an eccentricity of the FRP core 2 in the injection forming and the transfer forming. However, since a primer treatment is applied to the surface of the FRP core 2 for connecting the FRP core 2 to a rubber which forms the overcoat member 3, the operation for inserting the ring 56 through the FRP core 2 likely causes an adhesion of a fouling, which prevents a connection between the FRP core 2 and the overcoat member 3, on the surface of the FRP core 2. Moreover, in the method in which the support rod 53 is directly contacted to the FRP core 2, it is likely to make a flaw on the surface of the FRP core 2.
Further, since it is necessary to form, in the metal mold 54, the support rod 53 and a construction for taking the support rod 53 in and out, there is a problem such that the metal mold construction is complicated, and thus a maintenance and an operation are also troublesome. Furthermore, the rubber is arranged around the FRP core 2, which is set between the upper mold and the lower mold. The upper and lower molds are then closed around the FRP core. The above mentioned methods in which the support rod 53 is used have a technical disadvantage and the FRP rod 2 is liable to be damaged. Therefore, the above mentioned methods are not used in the compression forming method.
An object of the invention is to eliminate the drawbacks mentioned above and to provide an easier method of forming an overcoat member of a polymer insulator which prevents an eccentricity of the core member during a forming of the overcoat member without damaging the core member.
According to the method of forming the overcoat member of the polymer insulator of the invention, an overcoat member of a polymer insulator has a core member and is made of a sheath portion arranged around the core member. The overcoat member also includes a plurality of sheds and is formed by using a metal mold, comprising the steps of arranging a spacer for maintaining a thickness of the sheath portion during a forming operation to a portion of the metal mold corresponding to the sheath portion.
In the present invention, the spacer for maintaining a thickness of the sheath portion during a forming operation, which is arranged to a portion of the metal mold corresponding to the sheath portion, can prevent an eccentricity of the core member during a forming operation, so that it is possible to perform and overcoat member forming under such a condition that a thickness of the sheath portion is maintained constantly even. The spacer is preferably made of the same rubber as that of the overcoat member, and which is preferably projected inward in a cavity of the metal mold for forming the sheath portion under such a condition that the metal mold is set. The method according to the invention can be applied to all the known forming methods. However, if the method according to invention is especially applied to a compression forming, it is possible to achieve constructions such that a tip portion of the spacer is moved in response to a moving speed of the core member and that the tip portion of the spacer is constructed by a spacer head which can waggle its neck, so that it is a preferred embodiment.